When you open Sunday Keys inside MainStage, if the Tonic Pad Player has no sound when you press the on button, you'll want to make sure you are using MainStage 3.3 or later.
Apple added new functionality in the MainStage 3.3 release that makes Tonic possible, so it will not sound in previous versions of MainStage.
If you are using MainStage 3.3 or later, and are still having issues with the Tonic Pad Player, you will want to try a restart of your computer or download a fresh copy of Sunday Keys from inside My Purchases in your account. If you need a download link for Sunday Keys or run into any further issues, reach out to us here.